In its most basic form a light emitting diode (LED) comprises a light emitting layer which is positioned between an anode and a cathode. A hole injection layer may be incorporated between the anode and the light emitting layer (also known as the active or emissive layer). It functions to decrease the energy difference between the work function of the anode and the valence band or highest occupied molecular orbital (HOMO) of the light emitting layer, thereby increasing the number of holes introduced into the light emitting layer. Broadly speaking, in operation, holes are injected through the anode, and if present the hole injection layer, into the active layer, and electrons are injected into the active layer through the cathode. The holes and electrons combine in the light emitting layer radiatively to provide light. Equivalently, an electron injection layer between cathode and light-emitting layer can play the same role in controlling the injection of electrons into the light-emitting layer. A further role for these injection layers is to confine carriers within the device, so that under forward electric bias, electrons injected from the cathode into the light-emitting layer are significantly prevented from leaving this layer via the hole-injecting layer, and equivalently, holes injected into the light-emitting layer from the anode are significantly prevented from leaving this layer via the electron-injecting layer.
Some devices also incorporate a thin polymer interlayer between the hole injection layer and the light emitting layer. This plays an important role in improving the device efficiency and the lifetime of LEDs. For example, with an interlayer, blue light-emitting polymer organic light-emitting diodes (LEP OLEDs) with an external quantum efficiency of greater than 5% can be achieved, which is 35% higher than without the interlayer. It is believed that this may be due to the prevention of exciton quenching at the hole injection layer/light emitting layer interface.
Over the past two decades, solid state light-emitting devices based on direct bandgap semiconductors have been utilized as energy efficient sources of lighting. However, the fabrication of these devices typically relies on expensive high temperature and high vacuum processes, such as molecular beam epitaxy or thermal sublimation, rendering them uneconomical for use in large area displays.
Solution processing of luminescent semiconductors presents a particularly attractive option for the low-cost fabrication of light-emitting devices [see Burroughes et al. Nature 347, 539-541 (1990); Greenham et al, Nature 365, 628-630 (1993); Colvin et al, Nature 370, 354-357 (1994); and Coe et al, Nature 420, 800-803 (2002)]. Recent work on high-efficiency organometal halide perovskite photovoltaics has shown these materials to possess both the remarkable qualities of traditional semiconductors and the facile processability of organic semiconductors [Lee et al, Science 338, 643-647, doi:10.1126/science.1228604 (2012); Burschka et al., Nature 499, 316-319, doi:10.1038/nature12340 (2013); Liu et al., Nature 501, 395-398, doi:10.1038/nature12509 (2013); Stranks, et al., Science 342, 341-344, doi:10.1126/science.1243982 (2013); and Ball et al, Energy & Environmental Science 6, 1739-1743, doi:10.1039/c3ee40810h (2013)]. Further prior art can be found in, e.g. JP 2008-227330 A.
The semiconducting perovskite materials benefit from low cost and earth-abundance, and can be deposited at low temperatures under ambient conditions. More recently, bright and colour-controlled electroluminescence was reported in perovskite light-emitting diodes (PeLED), thereby opening up a potential range of display and lighting applications for these materials [see Tan, Z.-K. et al., Nat Nano 9, 687-692, doi:10.1038/nnano.2014.149 (2014)]. However, the quantum efficiencies in these devices remain modest due to difficulties in the formation of uniform thin films.
Light emission occurs when injected electrons and holes meet in the perovskite layer and recombine radiatively. However, it is easy for injected charges to bypass the semiconducting perovskite layer through pinholes in the thin films, leading to non-radiative current losses and a lower efficiency. Difficulties in the formation of uniform and pinhole-free semiconducting perovskites are well known, due to the material's crystalline nature. This problem is further exacerbated by the sublimation of excess methylammonium halide precursor during thermal annealing, thereby leaving voids in the perovskite layer. An established technique to overcome this problem involves sequential or vapor deposition of the perovskite precursors [see Liu et al above and Chen, Q. et al., Journal of the American Chemical Society 136, 622-625, doi:10.1021/ja411509g (2013)], although these methods only improve film formation and cannot completely eliminate pinholes.
There is therefore a need to provide an improved method for the preparation of semiconducting perovskite nanoparticle films which address this problem of the formation of pinholes in the films. The provision of such films will enable the manufacture of improved solid state light-emitting devices and other devices, e.g., solar cells, in which semiconducting perovskite films can be incorporated as a semiconductor. The luminescent nature of such films also makes them useful for emissive phosphors applications.